This invention concerns a weft thread clamp for a weaving machine containing two jaws that can be put in an open and closed position, with at least one jaw being directly moveable by fluid pressure to the closed position.
This invention also concerns a weaving machine equipped with one or more such weft thread clamps.
EP 1,266,986 describes a weft thread clamp with a pneumatically controlled jaw that contains an air bellows in which atmospheric pressure is built up to increase the volume of the air bellows in order to move a leaf spring to a clamping position. Thus, here this leaf spring functions as a means of clamping. Only the closing movement is controlled positively by this clamp. The movement to the opened position occurs by allowing the atmospheric pressure to decrease such that the opening of the clamp is done exclusively under the influence of the spring force of the leaf spring.
This weft thread clamp, however, has a disadvantage in that it has a limited reaction speed due to the time required for the air bellows to expand or contract. This makes it impossible during operation to guarantee that the clamp is always fully opened or closed at the points in time that an opened or closed position is required. Thus, the weft yarn regularly moves between jaws that are in an intermediate position between the closed and the opened position, causing increased friction between the yarn and the jaws. In addition, the late opening of the clamp causes greater tension to be built up in the yarn.
The increased friction causes more yarn particles and small fibres from the yarn to be released and deposed, among other places, on the weft thread clamp. This leads to a higher level of contamination. Higher yarn tension leads to more yarn breakage and thus more machine stoppages, resulting in reduced productivity and a greater risk of fabric irregularities that occur when the weaving machine is restarted.
Another disadvantage of the weft thread clamp according to EP 1,266,986 is that the air bellows is a component that is subject to wear. It is also not easy to durably and reliably assemble the air bellows using a heat shrink connection. Moreover, it is also extremely difficult to produce completely identical air bellows, meaning that the behaviour of the different air bellows under the influence of the same fluid pressure is often different. This requires an individual adjustment of the installed air bellows.
U.S. Pat. No. 4,494,579 describes a pneumatically controlled weft thread clamp in which the pressure difference in two chambers interconnected by a membrane is used to effect the movement of the means of clamping to the opened position. The closing movement is effected by a spring. A disadvantage of this weft thread clamp is that the closing movement of the means of clamping is not positively controlled, so that the clamping force cannot be influenced by the fluid pressure. Moreover, the reaction speed of this weft thread clamp is too slow to work with high weaving speeds. Another disadvantage is that two membranes must be provided, making this clamp quite complex. Because the required membranes are very sensitive to wear, there is a high risk of pressure loss with this type of weft thread clamp.
FR 2741332 discloses a thread clamp with the features mentioned in the first paragraph of this description. This clamp however takes up a lot of space. This is an important drawback, in particular in devices were a number of weft thread clamps must be installed one above the other.
The object of this invention is to provide a pneumatically controllable weft thread clamp that does not have the disadvantages indicated above.